This invention relates to a binder composition for binding foundry sand particles at the forming of molds and cores for use in a sand mold casting process and a coated sand prepared by using the same binder composition.
In current sand mold casting processes, molds and cores are usually formed of a resin coated sand, that is, by the use of a binder of which principal component is a thermosetting resin typified by phenolic resin to bind or integrate foundry sand particles. In iron casting processes, molds and cores formed of a resin coated sand are generally satisfactory both in high temperature strength and ease of disintegration after solidification of the poured molten iron.
However, the situation is different in some alloy casting processes characterized by relatively low pouring temperatures as typified by alluminum alloy casting processes wherein pouring temperatures are in the range of about 650.degree.-750.degree. C. Due to lowness of the pouring temperature, molds and cores of a resin coated sand retain their toughness even at the stage of shake-out and offer difficulties in disintegrating them. This problem is particularly serious for cores. It is a usual practice, therefore, to facilitate disintegration of the cores by baking cores in the castings at 400.degree.-500.degree. C. for a period of time as long as 4-10 hr in advance of a shake-out operation. This is of course unfavorable to the efficiency and costs of the casting process.
A primary reason for significantly higher resistance of cores to disintegration compared with molds is that, because the cores surrounded by the molten alloy undergo heating without being supplied with oxygen, the thermosetting resin used for binding the sand particles does not sufficiently decompose but undergoes significant carbonization which is adversely effective for lowering of the physical strength of the sand cores. In iron casting processes the same thermosetting resin undergoes sufficient decomposition owing to higher pouring temperatures (1300.degree.-1400.degree. C.) such that even cores exhibit sufficient lowering of physical strength and become readily disintegratable.
For molds, the degree of disintegratability does not become a serious problem because molds can readily be broken by externally applying mechanical force thereto. For cores, however, lack of disintegratability becomes a serious disadvantage since cores in the castings cannot easily be broken by the exertion of an external force. Accordingly wide studies have been made on binders for making sand molds and cores readily disintegratable after a casting process, but a fully satisfactory binder for this purpose has not yet been provided.
For example, it has been proposed to add a certain compound, such as potassium nitrate or sodium nitrate, which undergoes thermal decomposition with liberation of oxygen to a phenoric resin, a popular binder, expecting that the liberated oxygen will promote combustion of the phenoric resin in the cores heated during casting operation. Actually, however, the additive according to this method did not produce a practically appreciable improvement in the disintegratability of cores of the resin coated sand, so that this proposal has not been put into industrial practice. Besides, this method has disadvantages such as the tendency of lowering in the initial strength of the molds and cores and the presence of potassium oxide or sodium oxide formed by the decomposition of the additive or a hydroxide formed by reaction of such an oxide with water in the waste sand, causing the waste sand to become strongly alkaline and therefore making it necessary to neutralize the waste sand in advance of its reuse or dumping.
Also it has been proposed to replace a traditional phenolic resin by a more suitable resin, and modified phenolic resins have been subjected to industrial trial. However, hitherto proposed methods of this category are still unsatisfactory in the extent of improvement in the disintegratability of sand cores. From the same viewpoint, the use of an isocyanate known as the Ashland process is also unsatisfactory.